This invention relates to superconductors in general and more particularly to an improved method of manufacturing a superconductor.
Superconductive intermetallic compounds of the type A.sub.3 B with an A-15 crystal structure such as, for instance, Nb.sub.3 Sn or V.sub.3 Ga have very good superconduction properties and are distinguished, in particular, by a high critical magnetic field B.sub.c2, a high transition temperature T.sub.c and a high critical current density j.sub.c. Conductors with these materials are, therefore, suitable especially for superconducting magnet coils to produce strong magnetic fields. Besides these binary compounds, ternary compounds such as niobium-aluminum-germanium Nb.sub.3 Al.sub.0.8 Ge.sub.0.2 are also of special interest for conductors of such magnets.
However, these intermetallic compounds are generally very brittle, so that manufacturing them in a form suited, for instance, for magnet coils present difficulties. Special processes have therefore been developed, by which superconductors with these compounds can be produced in the form of long wires or ribbons. These processes allow, in particular, the manufacture of so-called multi-core conductors which contain a matrix of normally conducting material, in which wires of the compounds mentioned, e.g., Nb.sub.3 Sn or V.sub.3 Ga wires or niobium or vanadium wires, with surface layers of these compounds, are embedded. In such a process, a first component, which is in general a ductile element in wire form such as a niobium or vanadium wire, is surrounded by a jacket of a further component. This component consists, for instance, of a ductile carrier metal and an alloy containing the remaining elements of the compound, for instance, it consists of a copper-tin alloy or a copper-gallium alloy. A multiplicity of such wires can also be embedded in a matrix of the alloy. The assembly of these two components is then subjected to a cross-section reducing treatment. In this manner, a long wire is obtained such as is required for coils. In addition, the diameter of the wire cores, which consist of niobium or vanadium, is reduced with this treatment to a low value in the order of about 30 to 50 .mu.m or less, and the superconduction properties of the conductor ultimately produced are improved thereby. Furthermore, a good metallurgical bond is obtained by this process step between the wire cores and the matrix material of the alloy surrounding them, without the occurence of reactions that would embrittle the conductor. After the reduction of the cross-section, the intermediate conductor product of a superconductor, consisting of one or several wire cores and the surrounding matrix material, is then subjected to a heat treatment in such a manner that the desired superconductive compound is formed by a reaction of the core material with the other element of the compound which is contained in the surrounding matrix. The element contained in the matrix diffuses into the core material which consists of the other element of the compound and reacts with the latter, forming a layer consisting of the desired intermetallic compound British Pat. No. 1,280,583.
Superconducting magnet coils of such superconductors are generally produced according to two different methods. In the first method, which is also called the "wind-and-react technique", one first winds the not yet reacted intermediate conductor product of the superconductor on the coil form and then subjects the entire coil so wound to a diffusion anneal.
One thereby circumvents all difficulties in the processing of a brittle conductor material. It is also possible in this manner to make coils with a very small inside diameter with still relatively heavy conductors. In this method, however, all the materials required in the construction of the coil must be able to withstand the high temperature required for a diffusion anneal, which in the case of niobium-tin are above 700.degree. C., for several hours.
The second method for manufacturing superconducting magnet coils with the superconductors mentioned is the so-called "react first-then wind" method. With this method, the intermediate conductor product of the superconductor to be manufactured is wound on a temporary winding core and is then subjected to the required heat treatment for forming the superconductive compound. Subsequently thereto, the superconductor so produced is unwound from the winding core and can be processed further. In this process, however, there is danger of the turns being fritted together, and to the winding substrate, during the heat treatment. To prevent this fritting together, special separator means are necessary which must be arranged between the individual turns of the intermediate conductor product. Copper-beryllium and tantalum foils, for instance, are used as the separator means. However, these separator foils are relatively expensive. With them, there is also the danger of damage to the superconductor at butt joints at which individual parts of these foils are put together as well as the transitions from one layer to the next layer of the superconductor on the winding core in a multi-layer arrangement. Although this danger does not exist if separating means such as magnesium sludge U.S. Pat. No. 3,829,963 or a carbon coating U.S. Pat. No. 3,807,041 are used, the turns can touch at some points if these separating means are used, in case the turns become loose during the annealing operation. The turns then can frit together at these contact points. In addition, troublesome layers remain on the superconductor so produced, which only must be removed again in a secondary operation.
In addition, superconductors several kilometers long are often required for the construction of large magnets. Since, as is well known, solder joints between individual superconductor sections are sources of trouble, the requirement of large conductor lengths necessitates in general a multi-layer arrangement on the winding core for diffusion anneal, in which the individual layers of the intermediate conductor product must be separated from each other reliably.
The multi-core conductors manufactured by this second method are, in addition, also very sensitive to flexure and tension. For, their brittle A-15 phases are in general embedded in a soft material, e.g., bronze, which can take up only small mechanical loads. Excessive elongation of the conductor must therefore be avoided.
A further difficulty occurs due to a permanent length change of multi-core conductors produced by this method after the diffusion anneal, i.e., the turns applied lie only loosely on the respective substrate after the anneal, so that, in unwinding, the upper layers can slide into the ones below and can thus not be separated without damage.